void main(List<String> args) {
  String? str = 'this is str';
  str = null;

  print(str!.length);
}

/**
 *  Unhandled exception:
    Null check operator used on a null value
    #0      main (file:///e:/private_workspace/dart-learning-notes/dart_demo/9.%E6%96%B0%E7%89%B9%E6%80%A7/%E7%B1%BB%E5%9E%8B%E6%96%AD%E8%A8%80.dart:5:12)
    #1      _delayEntrypointInvocation.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:295:33)
    #2      _RawReceivePort._handleMessage (dart:isolate-patch/isolate_patch.dart:184:12)
 */